In recent years, photographic equipment such cameras and so forth have been popularized, and there have been increased photographic picture-taking opportunities with the use of a silver halide photographic light-sensitive material. Accordingly, there have been more serious demands for making a silver halide photographic light-sensitive material higher in sensitivity and image quality.
One of the dominant factors for making a silver halide photographic light-sensitive material higher in sensitivity and image quality is a silver halide grain. Therefore, research and developments of silver halide grains have so far been aimed at making sensitivity and image quality higher.
However, when making the grain size of silver halide grain smaller for improving image quality, their sensitivity is liable to be lowered, therefore, there has been a limitation in achieving both high sensitivity and high image quality compatible.
For trying to make both sensitivity and image quality higher, techniques for improving a sensitivity/grain size ratio per individual silver halide grain have been studied. As for one of the techniques, a technique in which tabular-shaped silver halide grains are used is described in, for example, Japanese Patent Publication Open to Public Inspection (hereinafter abbreviated to JP OPI Publication) Nos. 58-111935/1983, 58-111936/1983, 58-111937/1983, 58-113927/1983 and 59-99433/1984. When comparing these tabular-shaped silver halide grain to the so-called normal crystal silver halide grain having an octahedral, tetradecahedral or hexahedral crystal form, the former grain has a surface area wider than in the latter when the volume is the same. Therefore, the former grain has an advantage in making a sensitivity higher, because more sensitizing dyes can be adsorbed by the surfaces of silver halide grain.
Also, JP OPI Publication No. 63-92942/1988 describes a technique for putting a core having a high silver iodide content inside a tabular-shaped silver halide grain; JP OPI Publication No. 63-151618/1988 describes a technique for making use of a hexagonal tabular-shaped silver halide grain; JP OPI Publication No. 63-163451/1988 describes a technique for making use of a tabular-shaped silver halide grain having a ratio of the grain thickness to the farthest distance between twinned crystal faces of not less than 5; and each of the techniques display an effect on sensitivity and graininess, respectively.
Further, JP OPI Publication No. 63-106746/1988 describes a technique for making use of a tabular-shaped silver halide grain having a layered structure arranged to be parallel to two principal planes opposite to each other; and JP OPI Publication No. 1-279237/1989 describes a technique for making use of a tabular-shaped silver halide grain having a layered structure substantially partitioned by a plane parallel to two principal planes opposite to each other and also having an average silver iodide content of the outermost layer of at least not less than 1 mol % more than in the whole silver halide grain. Besides the above, JP OPI Publication No. 1-183644/1989 describes a technique for making use of a tabular-shaped silver halide grain having a completely uniform silver iodide distribution of a silver halide including silver iodide.
There are also various image-sharpness improving techniques of which are known in the art. One of the techniques is to prevent rays of light from scattering, and another is to improve an edge effect.
As for the latter techniques, these include a process for making use of a so-called DIR coupler and another process for making use of an unsharp mask. Among these processes, The latter has a practical limitation, because a sensitivity may be lowered and graininess may be deteriorated. As for the former process, numerous processes have been known so far and the useful DIR couplers include the compounds given in, for example, JP Examined Publication No. 55-34933/1980, JP OPI Publication No. 57-93344/1982 and U.S. Pat. Nos. 3,227,554, 3,615,506, 3,317,291 and 3,701,783. However, when emphasizing an edge effect by making use of a DIR coupler, MTF (standing for a modulation transfer function) may be improved in a low frequency range, but any MTF improvement cannot be expected in a high frequency range necessary for high magnification, and such an unfavorable side reaction as a lowered sensitivity and/or density may occur. When making use of a DIR coupler capable of displaying a functional effect over to a long distance, such as so-called diffusible DIR or timing DIR, sensitivity lowering and/or density lowering may be reduced, but an image-sharpness improvement in a highly enlarged image cannot be expected, because an MTF improvable region is further shifted to a lower frequency side.
On the other hand, as the techniques for preventing rays of light from scattering, a process for adding a colored substance, another process for reducing a silver halide content and a further process for thinning a layer have been known. A sharp reduction of a silver coated amount causes a graininess deterioration, because the numbers of dye clouds are reduced. There is a limitation to make a layer thinner by reducing gelatin, a coupler or a coupler solvent contained in a coating solution, because deterioration of coatability and color density are caused. An attempt has been tried since early times to prevent rays of light from scattering and to enhance an image sharpness by adding the foregoing colored substance. For example, there has been a known process for dyeing with a water-soluble dye so that irradiation may be prevented. However, this process is accompanied by an unfavorable side reaction such as a sensitivity lowering and, therefore, this process has been still unable to satisfactorily improve an MTF in a high frequency range.
On the other hand, as the techniques for making use of a grain effective to improve the image sharpness of a silver halide emulsion, there have been techniques for making use of a monodisperse core-shell type emulsion and a tabular-shaped core-shell type emulsion as disclosed in JP OPI Publication Nos. 60-138538/1985 and 60-143331/1985, U.S. Pat. No. 4,444,877 and JP OPI Publication Nos. 59-99433/1984 and 60-35726/1985. However, these techniques have still been unsatisfactory to display any improved effect.
In a silver halide photographic emulsion, that is prepared in the above-mentioned conventional techniques and is applicable to a common type light-sensitive material or a high image quality light-sensitive material, the preservation stability thereof has not been satisfactory for sensitivity, graininess and image sharpness. In particular, when making use of the tabular-shaped grains for effectively utilizing the merits thereof, the fluctuation in photographic properties do to aging them after carrying out a coating raise a problem. Therefore, the solution of the problem has been desired.